<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <style>
    *{
        margin: 0;
        padding: 0;
    }
  #box1 {
    margin: 20px;
    width: 200px;
    height: 200px;
    padding: 5px;
    border: 1px solid red;
    background-color: green;
    position: relative;
    top:30px;
    left:30px;
  }
  #box2{position: relative;
    margin:10px;
    width:300px;
    height: 300px;
    background-color: yellow;
  }
#box3{
    position:absolute;
    width:50px;
    height: 50px;
    background-color: pink;
    top:10px;
    left: 10px;
}
  </style>
</head>
<body>
  <!-- 
    页面绘制出来
    box1 在box2上面， 正常的文档流
    html 文档流 和 css 结合 样式生效 盒模型
  -->
  <div id="box1"></div>
    <div id ="box3">box3</div>
  <div id="box2"></div>
</body>
</html>
